1. Field of the Invention
The present invention relates to a liquid crystal display panel using an active matrix substrate, and more particularly to the structure of an active matrix substrate.
2. Description of the Related Art
Liquid crystal display panels have been finding widespread use primarily in computer displays and in other fields ranging from audio visual apparatus such as projectors to amusement equipment such as game machines. It is desired to develop liquid crystal panels with higher resolution to meet the requirements of a variety of media.
FIG. 5 is a plan view of an active matrix substrate used in a conventional liquid crystal display panel; one pixel section of a plurality of identical pixel sections is shown here. FIG. 7 is a schematic circuit diagram showing the circuit structure for one pixel.
Referring to FIG. 5, the active matrix substrate 201 includes a quartz substrate 1 on which a plurality of liquid crystal driving electrodes (hereinafter called the pixel electrodes) 54 for driving the liquid crystals are arranged in a checkerboard pattern. A data signal line 11a for supplying data to the pixel electrode 54 and a data signal line 11b for supplying data to an adjacent pixel electrode are shown formed on the quartz substrate 1. The supply of data is controlled by a switching device 13 constructed from a thin film transistor (hereinafter abbreviated TFT). The quartz substrate 1 also has a plurality of scanning signal lines 12 arranged thereon intersecting at right angles with the data signal lines 11a and 11b. A portion of one scanning signal line 12 functions as the gate of the switching device 13 whose source 13a is connected to the data signal line 11a through a contact hole 3a and whose drain 13b is connected to the pixel electrode 54 through a contact hole 3b.
A storage capacitance (C.sub.S) 23 forms a pixel Capacitance (C.sub.F) 34 together with a liquid crystal capacitance (C.sub.LC) formed between the pixel electrode 54 and a counter substrate electrode (not shown). A coupling capacitance (C.sub.SD1) 35 is formed between the pixel electrode 54 and the data signal line 11a; a coupling capacitance (C.sub.SD2) 36 is likewise formed between the pixel electrode 54 and the data signal line 11b connected to the adjacent pixel electrode.
The source, drain, and channel of the TFT 13 are formed from portions of a semiconductor layer 113 formed on the quartz substrate 1, while the scanning signal lines 12 and one common electrode 123 of the storage capacitance 23 are formed from, for example, a polysilicon layer doped with Phosphorus formed above the semiconductor layer 113 with an interlevel insulating film interposed therebetween. The portion 13c of the semiconductor layer 113 that faces the one electrode 123 functions as the other electrode of the storage capacitance. The data lines 11a and 11b are formed from an aluminum layer deposited above the layer of the common electrode with an interlevel insulating film interposed therebetween, and the pixel electrode 54 is formed from an indium tin oxide film (hereinafter sometimes referred to as the ITO film) formed above the aluminum layer with an interlevel insulating film interposed therebetween.
An operational description will be given below.
When the TFT 13 is selected by a scanning signal supplied from the scanning signal line 12 and placed in a conducting state, the data signal supplied from the data signal line 11a flows through the TFT 13 and is written into the pixel capacitance 34. Data are written into the pixel capacitance 34 connected to each scanning signal line 12 in this manner, until one picture field or frame is completed.
However, in the conventional active matrix substrate 201 shown in FIG. 5, the direction of the electric field being applied to the liquid crystals on the pixel electrode 54 is disturbed by the effects of different potentials from the data signal lines 11a, 11b and scanning signal lines 12. This results in the disruption of liquid crystal orientation, causing disclination and, in the case of a normally white display, light leakage and hence reduced contrast. Conventionally, this problem has been addressed by masking the disrupted portion of the liquid crystal orientation with a black matrix formed on the counter substrate to prevent display quality degradation.
In Japanese Patent Publication No. 4-74714 and Japanese Patent Laid-Open Publication NOS. 63-301924 and 58-172685, there is proposed a structure for solving the problem of the above active matrix substrate 201, in which a portion of the pixel electrode 64 is overlaid on the data signal line 11a with an insulating film sandwiched therebetween, as exemplified in FIG. 6, thereby reducing the disruption of the electric field. In the overlaid portion of the pixel electrode, the direction of the electric field on the pixel electrode due to the potential of the data signal line is perpendicular to the pixel electrode, which serves to suppress the disruption of the liquid crystal orientation on the pixel electrode.
The conventional active matrix substrate 201 has had a problem of crosstalk. When the TFT 13 is in a nonconducting state and the data signal written in the pixel capacitance 34 is retained there, the pixel potential is affected by data signals via the coupling capacitances 35 and 36; therefore, in a generally practiced driving method wherein the data signal polarity is inverted between fields or between frames, if either or both of the coupling capacitances 35 and 36 are appreciably greater than the pixel capacitance 34, crosstalk occurs in vertical directions. To overcome this problem, a method is known which drives the liquid crystals by inverting the data signal polarity between data signal lines (refer to Japanese Patent Publication No. 5-43118). According to this driving method, since the polarity of the data signal applied to one data signal line is different from that applied to the next data signal line, the variation of the pixel potential due to the data signal is cancelled out.
However, in the conventional active matrix substrate 202 of the structure shown in FIG. 6, the coupling capacitance C.sub.SD1 between the pixel electrode 64 and its corresponding data signal line 11a becomes greater than the coupling capacitance C.sub.SD2 between the pixel electrode 64 and the data signal line 11b connected to the pixel electrode in the adjacent column, and it is not possible to sufficiently cancel out the effect caused on the pixel electrode from the adjacent data signal line to which a signal of inverted polarity is applied. With the above driving method therefore, it is not possible to sufficiently eliminate crosstalk. Accordingly, in the conventional active matrix substrate, it has not been possible to prevent the disruption of the liquid crystal orientation and the occurrence of crosstalk at the same time, in developing a higher resolution liquid crystal display panel.